1. Field of the Invention
The present invention relates to a liquid crystal panel and a liquid crystal display using the panel. In particular, it relates to a liquid crystal panel and a liquid crystal display which is driven in a mode which aligns liquid crystal molecules having a negative dielectric anisotropy almost perpendicular to the substrates of the panel in the absence of applied voltage (vertical aligned mode, referred to as VA mode below).
2. Description of the Related Art
Liquid crystal displays are used widely as a display for various information processing devices, such as computers and TVs.
In a conventional liquid crystal display, a liquid crystal panel of a TN type, an STN type or a TFT type etc. is widely used in a mode which aligns the liquid crystal molecules having a positive dielectric anisotropy, i.e., a p type liquid crystal, parallel to the substrates opposed to each other in the absence of applied voltage.
In a liquid crystal panel of a TN mode, for example, the alignment direction (the parallel direction) of the liquid crystal molecules adjacent to one substrate is usually twisted 90 degrees to that of the liquid crystal molecules adjacent to another substrate. When a pair of polarizing plates disposed outside of the panel are arranged so that their transmission axes intersect at right angles with each other (crossed nicol), a white display is realized in a non-driving state, while a black display is realized in a driving state. On the contrary, when the transmission axes of the polarizing plates are arranged parallel to each other (parallel nicol), a black display is realized in a non-driving state while a white display is realized in a driving state.
In such a TN mode crystal display panel, it is understood that the liquid crystal molecules are aligned parallel to the substrate surface of the panel in a non-driving state, and on the contrary, they are aligned almost perpendicular to the substrate surface of the panel in a driving state. In reality, however, the liquid crystal molecules adjacent to the substrate surface of the panel keep the parallel alignment even in a driving state. The liquid crystal molecules in such a parallel alignment cause a birefringence which transmits some amount of light through the panel in a driving state. Due to this fact, a high contrast cannot be realized in the conventional TN mode liquid crystal panel.
As shown in Unexamined Japanese Patent Publication No. 8-43825, VA mode liquid crystal panels are being developed for practical application. In a VA mode liquid crystal panel, a liquid crystal having a negative dielectric anisotropy is held between a pair of substrates forming the panel in a manner that the molecules of the liquid crystal are aligned almost perpendicular to the substrates.
Since the liquid crystal molecules in this VA mode panel are aligned almost perpendicular to the substrate surface in a non-driving state, incident light passes through the liquid crystal layer without its polarization plane changing very much. Thus, when the transmission axes of a pair of polarizing plates provided to sandwich the substrates are arranged perpendicular to each other (crossed nicol), an almost perfect black display can be realized in a non-driving state. Contrary to this, a white display can be realized in a driving state due to the rotation of the polarization plane of the incident light, since the liquid crystal molecules move to align parallel to the substrates while retaining the 90 degree twisted relation between the two substrates.
As a result, a very high contrast can be obtained in such a VA mode liquid crystal panel, a level of contrast which cannot be obtained by a TN mode liquid crystal panel.
However, the VA mode liquid crystal panel has a problem that it is colored yellow when observed from an upper oblique direction. To avoid this problem, it is possible to decrease the retardation of a liquid crystal panel. However, the display contrast of the panel deteriorates as the retardation of the panel decreases, thus reducing the brightness of the panel.